This invention relates generally to the laser art and more particularly to solid state, room temperature, continuous wave lasers and the method of operation thereof.
It is well known that some rare earth ions, when incorporated as impurities in sufficient concentration into a suitable host lattice, can upconvert infrared radiation to various shorter wavelengths. Upconversion, i.e., the conversion of long-wave into short-wave radiation by certain solids, without assistance of auxiliary radiation, can be accomplished by several multiphoton mechanisms. However, only one, the cooperative excitation mechanism, is thought to be efficient enough to be practical. The latter mechanism proceeds according to a scheme in which the groundstate electrons of several atoms (ions) absorb one infrared photon each. The energy subsequently migrates through a nonradiative process to a single atom exciting it to a higher energy level with ensuing fluorescence.
Approximately 15 transition group ions, primarily rare-earths, incorporated in various solid-state host materials are known to lase in a continuous wave (CW) mode but a majority of these lines require low temperature for lasing. Room temperature CW operation is important for many scientific, medical and industrial applications. For example, CW lasers are used for spectroscopic studies, for surgical and coagulating purposes, for cutting and drilling materials, for communication in conjunction with integrated optics and alignment of electro-optical hardware.
Er.sup.3+ ions have been lased in solid state, room temperature lasers but in a pulsed mode only. Lasing of Er.sup.3+ ions between .sup.4 I.sub.11/2 and .sup.4 I.sub.13/2 states (See FIGS. 2 and 11) around 3 .mu.m in a pulsed mode has been produced in the past in more than a dozen different host materials. In many hosts, Er.sup.3+ lased without deactivating ion assistance.
Most 3 .mu.m erbium lasers have the following properties in common: (1) they operate at room temperature; (2) they require high erbium ion concentration for low excitation energy; and (3) they are characterized by the terminal state/initial state lifetime ratio greater than one, in some materials this ratio exceeds ten. Surprisingly, these lasers operate quite well in a pulsed mode at room temperature with low pump energy threshold. The third property would seem to prohibit these lasers from lasing in a CW mode.
To explain pulsed lasing, a hypothesis was put forward by previous workers taking into account Stark splitting of the .sup.4 I.sub.11/2 and .sup.4 I.sub.13/2 excited states. According to the hypothesis, the .sup.4 I.sub.11/2 .fwdarw..sup.4 I.sub.13/2 lasing process proceeds as follows. Initially, the upper levels of the terminal state are sparcely populated. Therefore, population inversion is produced at the levels of .sup.4 I.sub.11/2 manifold resulting in The shorter wavelength lines rapidly disappear from the laser spectrum because the lower lying levels of the terminal manifold saturate faster. Only the longest wavelength lines, terminating on the upper levels of the terminal manifold, would survive and be observed in the late laser spectrum. This is the so-called "red shift" of spectral lines of self-saturating lasers. Laser transitions between states, for which initial state lifetime is shorter than terminal state lifetime, are called self-saturating.